System and method for anticipatory receiver switching based on signal quality estimation

ABSTRACT

In various embodiments, a first and second complex multiplier may be configured to receive an input signal and provide a baseband I component signal and a baseband Q component signal, respectively. A first and second filter may be configured to filter the baseband I component signal and the baseband Q component signal, respectively. An equalizer may be configured to equalize the filtered baseband I component signal and the filtered baseband Q component signal. A carrier recovery portion may be configured to generate a reference signal based on the equalized filtered baseband I component signal and the equalized filtered baseband Q component signal. A first and second multilevel comparator may be configured to receive the equalized filtered baseband I component signal from the carrier recovery portion and provide an output I and receive the equalized filtered baseband Q component signal and provide an output Q signal for further modulation.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a continuation of, and claims priority under35 U.S.C. §120 to, U.S. patent application Ser. No. 12/503,805, filedJul. 15, 2009, entitled “System and Method for Anticipatory ReceiverSwitching Based on Signal Quality Estimation,” which is a continuationof, and claims priority under 35 U.S.C. §120 to, U.S. patent applicationSer. No. 11/452,216, filed Jun. 14, 2006, now U.S. Pat. No. 7,570,713,entitled “System and Method for Anticipatory Receiver Switching Based onSignal Quality Estimation,” each of which is incorporated herein byreference.

BACKGROUND

Radio communication systems are becoming more reliable, and the MeanTime Between Failure (MTBF) associated therewith is high. However, inmicrowave radio transmission, the associated transmission link may belong and multipath may be frequently encountered. Multipath refers tomultiple transmission paths between transmit and receive antennas of acommunication system. Multipath may result in both frequency-selectivefading and space-selective fading. Frequency-selective fading generallyindicates that a channel varies with frequency. Space-selective fadinggenerally indicates that a channel is dependent upon the position of therespective transmit and receive antennas. When multipath interferes withreception of a radio transmission signal, the received signal isdistorted causing errors in the corresponding demodulated data stream.

FIG. 1A illustrates an arrangement of symbol points in accordance with a4 QAM (Quadrature Amplitude Modulation) method on an I-Q coordinateplane. With reference to FIG. 1A, a symbol point of a received signalcorresponds to any of four signal points positioned concentrically onthe I-Q coordinate plane. Therefore, it is possible to transmit at onetime 2 bits of data representing any of the four signal points.

Advances in radio communication systems, however, require datatransmission of larger volume at higher speeds. Accordingly, multiplevalue (M-ary) modulation methods having values larger than the 4 QAMmodulation method described above have been developed. As an example ofsuch an M-ary modulation method, 16 QAM is commonly employed in datacommunications. FIG. 1B is an illustration of an arrangement of symbolpoints in accordance with the 16 QAM modulation method on the I-Qcoordinate plane. With reference to FIG. 1B, a symbol point of areceived signal corresponds to any of a total of 16 signal points on thecoordinate plane, arranged four by four in a lattice form in eachquadrant of the I-Q coordinate plane. Therefore, it is possible totransmit at one time 4 bits of data representing any of the 16 signalpoints.

When a modulation method having a larger M-ary number is employed andthe communication environment of the propagation path is defective(i.e., if the propagation path has severe interference, noise orencounters multi-path), symbol points may be recognized erroneouslysince the interval between each of the symbol points is narrow and thesymbol points are arranged tightly in the respective modulation method,as may be seen from the arrangement of symbol points of FIG. 1B.Therefore, though this method has a communication speed faster than the4 QAM modulation method illustrated in FIG. 1A, it is more susceptibleto reception errors.

In a radio communication environment prone to multipath, several knowntechniques may be implemented to mitigate the effects of multipath.Prior art techniques commonly used to protect a signal path includeswitching from an online channel, receiver or antenna to a standbychannel, receiver or antenna by 1+1 Frequency Diversity (FD) or 1+1Space Diversity (SD) or combination thereof.

SD may commonly be provided by utilizing multiple receive antennasseparated by a sufficient distance to take advantage of space-selectivefading. With reference to FIG. 2, a prior art method is illustrated thatutilizes SD with two receive antennas including a separate receiverconnected to each receive antenna. A pair of antennas 201, 202 arecoupled to respective receivers 203, 205 that demodulate the signalsreceived at each antenna. An antenna selection circuit 209 accepts thedemodulated output of the receivers and provides control to an antennaswitch 207 to select a data set having the least amount of error.

The aforementioned diversity techniques and examples, however, are notalways appropriate for evaluating the communication quality of thepropagation path. For example, different radio reception apparatusesemploy different methods of reception and performances. Qualities ofcomponents, such as filters used in the reception apparatuses, vary andsuch differences and variations have an influence on the quality ofcommunication. Conventional parameters such as reception level, frameerror rate, and carrier to interference ratios (CIR) do not reflect suchquality or performances of the reception apparatuses. Further, as may beseen from a comparison of the modulation methods shown in FIGS. 1A and1B, even when there is no reception error with a modulation methodhaving smaller M-ary values, it is unpredictable whether there arisesreception error or not with another modulation method (16, 64, 128, 256QAM) having a larger M-ary values (i.e., having dense symbol points onthe I-Q plane). As a result, special and complicated procedures andhardware are necessary to measure the respective conventional parametersof the propagation path during communication.

Thus, there is a need in the art for a system and method of selectingantennas or receivers in a multipath environment without incurring anyerrors in the received signal.

Accordingly, it is an object of the present subject matter to obviatemany of the deficiencies in the prior art and to provide a novel methodof switching from a first receiver receiving a constant bit rate signalto a second receiver receiving the constant bit rate signal, where theconstant bit rate signal received by the first and second receivers isconverted to a first baseband signal and a second baseband signal. Themethod further comprises the steps of estimating a signal quality metricof the first baseband signal, comparing the signal quality metric to apredetermined threshold, and switching from the first baseband signal tothe second baseband signal if the signal quality metric is greater thanthe threshold.

It is also an object of the present subject matter to provide a novelmethod of switching from a first receiver receiving a constant bit ratesignal to a second receiver receiving the constant bit rate signal,where the constant bit rate signal received by the first and secondreceivers is converted to a first baseband signal and a second basebandsignal, respectively. The method further comprises the steps ofestimating a first signal quality metric of the first baseband signal,estimating a second signal quality metric of the second baseband signal,comparing the first signal quality metric to the second signal qualitymetric, and switching from the first baseband signal to the secondbaseband signal if the first signal quality metric is greater than thesecond signal quality metric.

It is another object of the present subject matter to provide a novelsystem for switching from a first receiver receiving a constant bit ratesignal to a second receiver receiving the constant bit rate signal. Thesystem comprises a first converting circuit for converting the signalfrom the first receiver to a first baseband signal and a secondconverting circuit for converting the signal from the second receiver toa second baseband signal. The system further comprises an estimatingcircuit for estimating a first signal quality metric of the firstbaseband signal, a comparing circuit for comparing the first signalquality metric to a predetermined threshold, and a switching circuit forswitching from the first baseband signal to the second baseband signalif the first signal quality metric is greater than the predeterminedthreshold.

It is still an object of the present subject matter to provide a novelsystem of switching from a first receiver receiving a constant bit ratesignal to a second receiver receiving the constant bit rate signalcomprising a first converting circuit for converting the signal from thefirst receiver to a first baseband signal and a second convertingcircuit for converting the signal from the second receiver to a secondbaseband signal. The system also comprises a first estimating circuitfor estimating a first signal quality metric of the first basebandsignal, a second estimating circuit for estimating a second signalquality metric of the second baseband signal, a comparing circuit forcomparing the first signal quality metric to the second signal qualitymetric, and a switching circuit for switching from the first basebandsignal to the second baseband signal if the first signal quality metricis greater than the second signal quality metric.

These and many other objects and advantages of the present inventionwill be readily apparent to one skilled in the art to which theinvention pertains from a perusal of the claims, the appended drawings,and the following detailed description of the preferred embodiments.

SUMMARY OF THE INVENTION

In various embodiments, a system comprises a first and second complexmultiplier, a first and second filter, an equalizer, a carrier recoveryportion, and a first and second multilevel comparator. The first complexmultiplier may be configured to receive an input signal and provide abaseband I component signal. The second complex multiplier may beconfigured to receive the input signal and provide a baseband Qcomponent signal. The first filter may be configured to filter thebaseband I component signal. The second filter may be configured tofilter the baseband Q component signal. The equalizer may be configuredto equalize the filtered baseband I component signal and to equalize thefiltered baseband Q component signal. The carrier recovery portion maybe configured to generate a reference signal based on the equalizedfiltered baseband I component signal and the equalized filtered basebandQ component signal. The first multilevel comparator may be configured toreceive the equalized filtered baseband I component signal from thecarrier recovery portion and provide an output I signal for furtherdemodulation. The second multilevel comparator may be configured toreceive the equalized filtered baseband Q component signal from thecarrier recovery portion and provide an output Q signal for furtherdemodulation.

The system may further comprise a lookup table (LUT) wherein the LUTprovides a sine signal to the first complex multiplier and a cosinesignal to the second complex multiplier. The sine signal and the cosinesignal may be based on the reference signal from the carrier recoveryportion. Further, the sine signal may be substantially the same carrierfrequency and is in phase with the input signal and the cosine signalmay be substantially the same carrier frequency and is in quadraturephase with the input signal.

The system may also further comprise a signal quality estimation circuitconfigured to estimate a signal quality based on the filtered baseband Icomponent signal and the filtered baseband Q component signal. Thesignal quality estimation circuit may be configured to estimate a signalquality based on the equalized filtered baseband I component signal andthe equalized filtered baseband Q component signal. Further, the signalquality estimation circuit may be configured to estimate a signalquality based on the equalized filtered baseband I component signalreceived form the carrier recovery portion and the equalized filteredbaseband Q component signal received from the carrier recovery portion.

The signal quality estimation circuit may comprise a distancecalculator, an average calculator, and a comparator. The distancecalculator may be configured to determine a distance between an idealsymbol and a received symbol of the equalized filtered baseband Icomponent signal and the equalized filtered baseband Q component signal.The average calculator may be configured to determine an averagedistance between based on the distance from the distance calculator. Thecomparator may be configured to compare the average distance to apredetermined threshold. The signal quality estimation circuit may befurther configured to generate an alert signal when a quality is lowbased on the comparison of the average distance to the predeterminedthreshold.

In various embodiments, a method may comprise multiplying an inputsignal with a sine signal to provide a baseband I component signal,multiplying the input signal with a cosine signal to provide a basebandQ component signal, filtering the baseband I component signal, filteringthe baseband Q component signal, equalizing the filtered baseband Icomponent signal, equalizing the filtered baseband Q component signal,generating a reference signal based on the equalized filtered baseband Icomponent signal and the equalized filtered baseband Q component signal,comparing the equalized filtered baseband I component signal to providean output I signal for further demodulation, and comparing the equalizedfiltered baseband Q component signal to provide an output Q signal forfurther demodulation.

A system may comprise a means for multiplying an input signal with asine signal to provide a baseband I component signal, a means formultiplying the input signal with a cosine signal to provide a basebandQ component signal, a means for filtering the baseband I componentsignal, a means for filtering the baseband Q component signal, a meansfor equalizing the filtered baseband I component signal, a means forequalizing the filtered baseband Q component signal, a means forgenerating a reference signal based on the equalized filtered baseband Icomponent signal and the equalized filtered baseband Q component signal,a means for comparing the equalized filtered baseband I component signalto provide an output I signal for further demodulation, and a means forcomparing the equalized filtered baseband Q component signal to providean output Q signal for further demodulation.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1 a and 1 b illustrate a representation of arrangements of symbolpoints of 4 QAM and 16 QAM, respectively, on an I-Q coordinate plane.

FIG. 2 is a simplified block diagram of a prior art receiving apparatusthat includes two receivers and compares the received signals afterdemodulation to select an antenna for reception.

FIG. 3 is a simplified block diagram of a signal quality estimationcircuit according to an embodiment of the present subject matter.

FIG. 4 is a functional block diagram of a signal quality estimationmethod according to an embodiment of the present subject matter.

FIG. 5 is an illustration of the ideal constellation points of a 4 QAMmodulation method having a received symbol in the (1,1) quadrant.

FIG. 6 is a simplified block diagram of a 1+1 SD configuration accordingto an embodiment of the present subject matter.

FIG. 7 is a simplified block diagram of a 1+1 FD configuration accordingto an embodiment of the present subject matter.

DETAILED DESCRIPTION OF THE FIGURES

With reference to the figures where like elements have been given likenumerical designations to facilitate an understanding of the presentinvention, the various embodiments of a system and method for switchingfrom a first receiver to a second receiver without error in a receivedsignal by utilizing signal quality estimation are described. Switchingaccording to embodiments of the present subject matter may be conductedin conjunction with 1+1 or 1 for N protected radio links or othertransmission media to thereby protect against signal path disturbance,equipment degradation or equipment failure and to achieve high dataavailability carried through such media. Furthermore, embodiments of thepresent subject matter estimate signal quality of a received signalderived from the decision directed coherent demodulation ofM-ary/M-state QAM/PSK (i.e., 4 QAM (PSK) to 256 QAM (PSK) or higher).

A system and method according to the present subject matter may utilizean exemplary signal quality estimation circuit illustrated by FIG. 3.With reference to FIG. 3, a functional block diagram of a front endportion of a demodulator comprising a demodulator circuit 300 isillustrated in accordance with an embodiment of the present subjectmatter. While the demodulator circuit 300 is described herein withapplication to M-ary quadrature amplitude modulation (QAM), it is to beunderstood that the demodulator circuit 400 may be utilized with varioustypes of demodulators such as those applicable to phase shift keying(PSK) and multiple values thereof (i.e., M-state), quadrature phaseshift keying (QPSK), as well as QAM, and multiple values thereof (i.e.,M-ary).

The demodulator circuit 300 receives a modulated IF signal 310. Themodulated IF signal 310 is provided to complex multipliers 320, 322which provide baseband I and Q component signals 324 and 326,respectively. A sine/cosine look-up table (LUT) 330 provides sine andcosine signals, sin(x) and cos(x), 316 and 318 respectively, to thecomplex multipliers 320, 322. The phase components of the sine andcosine signals are such that the multiplication of the modulated IFsignal 310 to form I and Q components followed by an appropriatefiltering 340, 341 (i.e., low pass, loop or other well known filters)provide I and Q signals 342 and 344, respectively.

The signals 342, 344 are provided to an equalizer 350 which processesthe signals and passes equalized signals to a carrier recovery portion360 of the demodulator circuit 300. Broadly, the equalizer 350 processesand/or filters the received signals 342, 344 to reduce or eliminateintersymbol interference (ISI). An equalizer according to an embodimentof the present subject matter may conduct maximum likelihood sequenceestimation, filter equalization (i.e., linear, non-linear filters), orother forms of equalization known in the art. The carrier recoveryportion 360, as is known in the art, creates a reference signal forinput into the LUT 330 which, in turn, outputs a signal 316 having afrequency that is substantially the same as the carrier frequency and isin phase with the carrier signal, and outputs a signal 318 having afrequency that is substantially the same as the carrier frequency but isout of phase (quadrature) with the carrier signal. The carrier recoveryportion 360 provides equalized I and Q signals 362 and 364,respectively, from the equalizer 350 to multilevel comparators 372, 374and a signal quality estimation circuit 376. Of course, the demodulatorcircuit 300 may include various other components such as a slopeequalizer, a DC offset compensation circuit, converters, gaincompensation circuits, clock recovery circuits, demapper circuits,forward error correction circuits, and combinations thereof, as well asother demodulator components known in the art. Furthermore, the sensingconnections for the signal quality estimation circuit 376 may be movedto the output of the filters 340, 341 or the output of the equalizer350. The demodulator circuit 300 may thus accurately estimate the signalquality for different modulation levels and different FEC coding schemessuch as 2D TCM, 4D TCM, Reed Solomon code, and concatenation of TCM andReed Solomon code. Of course, the demodulator circuit 300 may beimplemented through hardware or software or a combination of bothhardware and software.

FIG. 4 is a functional block diagram of a signal quality estimationcircuit 376 according to an embodiment of the present subject matter.With reference to FIG. 4, an estimation circuit continuously monitorsthe quality of the received signals 362, 364 provided by the carrierrecovery portion 360 and declares an alarm 434 when the quality is belowa certain level. As represented by block 410, the signal qualityestimation circuit determines the distance between a received symbol andan ideal symbol that would have been received if there was no impairmentof the signal. This impairment may be the result of thermal noise,signal distortion due to multipath fading (selective fading),interference, etc. As represented by block 420, the signal qualityestimation circuit determines an average distance between the distancesbetween successive received symbols and the corresponding ideal symbols.As represented by block 430, the signal quality estimation circuitcompares the average distance to a predetermined threshold.

For example, FIG. 5 illustrates ideal constellation points of a 4 QAMconstellation with a received symbol 502 in the (1,1) quadrant. A signalquality estimation circuit according to an embodiment of the presentsubject matter may determine a signal quality metric as a function ofthe distance (d) between the received symbol location 502 and apredetermined ideal symbol location 504. For example, the higher thedistance (d), the lower the quality of the baseband signal. Bymonitoring successive received symbols, determining the distance betweenthe successive received symbols and the predetermined ideal symbollocation associated therewith 410, the estimation circuit may determinean average distance 420 for the measured distance determinations andcompare 430 the average distance against a predetermined threshold 432.If the predetermined threshold 432 is exceeded then the estimationcircuit produces an alarm 434. A further embodiment of the signalquality determination according to the present subject matter comprisesdetermining, for each of n symbols of a signal, a distance between alocation of the symbol and an associated predetermined ideal symbollocation, determining a sum of the distances for the n symbols, anddividing the sum by n. The sum divided by n will be compared to thepredetermined threshold 432 and an alarm produced 434 if thepredetermined threshold is exceeded.

FIG. 6 is a simplified block diagram of a communications systememploying a 1+1 SD configuration according to an embodiment of thepresent subject matter. With reference to FIG. 6, a communicationssystem 600 employing a 1+1 SD configuration is illustrated where aconstant bit rate signal or RF carrier is being transmitted while twoantennas 604, 606 are utilized to receive the signal. Each antennaprovides its respective received signal to an associated receiver 610,612. The antennas 604, 606 may be physically displaced, to therebyachieve a space diversity effect. Any of the receivers may operate in afull duplex communication system and may also comprise part of acommunication node that transmits and receives constant bit rate data ina full duplex mode with other communication nodes (not shown).

FIG. 7 is a simplified block diagram of a communications systememploying a 1+1 FD configuration according to an embodiment of thepresent subject matter. With reference to FIG. 7, a communicationssystem 700 employing a 1+1 FD configuration is illustrated where data istransmitted redundantly through two different carriers via a singleantenna 704. The antenna 704 provides a received constant bit ratesignal to a splitter circulator 702 which splits and distributes thesignal to associated receivers 610, 612. Any of the receivers mayoperate in a full duplex communication system and may also comprise partof a communication node that transmits and receives constant bit ratedata in a full duplex mode with other communication nodes (not shown).

With reference to FIGS. 6 and 7, modulated signals are provided by thereceivers 610, 612 to associated demodulators 620, 622 to recover anddemodulate the data. The signals may be converted to baseband signals incomponents of the demodulators 620, 622 or by a separate convertingcircuit (not shown). After further signal processing, the two redundantdata streams may be provided by the demodulators 620, 622 to aDifferential Absolute Delay Equalizer (DADE) circuit 640. Alarminformation provided by the signal quality estimation circuit 376 fromthe respective demodulator may be supplied to a radio controller orcontrol processing unit (CPU) 630. Embodiments of the CPU 630 may pollthe alarm signal 434 provided by the signal quality estimation circuit376 at predetermined intervals, e.g., 1 msec, 5 msec, 10 msec. Ofcourse, the CPU 630 may receive a continuous signal from the alarmsignal 434 or may poll the alarm signal 434 at different intervals thanthose identified and such examples are not intended to limit the scopeof the claims appended hereto. The CPU 630 utilizes the alarminformation alone or in combination with other information such as IFspectrum or signal slope information, BER information, signal degradeinformation based on data derived from FEC decoding processes, andsignal outage information to provide a switching decision to the DADEcircuit or another suitable switching circuit based on a presetswitching request priority. A switching request priority for a receiverswitching algorithm according to an embodiment of the present subjectmatter, listed from highest priority to lowest priority, may comprise:force switched, BER or frame loss alarm, signal degrade alarm, IF slopealarm, and receiver signal level (RSL) low alarm sensed at a radioantenna port.

The DADE circuit 640 compensates for any delay between the two datastreams provided by the receivers and aligns the data streams so that adata transition from an active channel to a standby channel occurswithout error. For example, when switching occurs, the two data streamsprovided by the receivers must be aligned so that the transition of datafrom one receiver to the data from a second receiver occurs withouterror. The DADE circuit may comprise two delay flip-flop buffers. Onebuffer delays data signals from one receiver and the second bufferdelays data signals from the second receiver. Since the signal carriedby the two receivers possesses disparate paths (i.e., the signals passthrough different hardware), there exists some delay between the twosignals. The DADE circuit may constantly monitor and align the two datastreams from the two receivers. The active channel or receiver bufferwill be read and outputted at the center of the buffer while the datafrom the standby channel or receiver is altered to possess the samedelay as that of the active channel or receiver. The DADE circuit mayalso dynamically correct the delay alignment if one or both receiversexperience a multipath (selective) fading that sifts the data around.When switching occurs, the selected buffer outputs the data to thefollowing circuit for further processing. Since the two incoming signalsto the DADE circuit possess different delay and phase, the read clockfor both buffers is synchronized by the incoming clock of the selectedchannel. In the event of a sudden phase jump of the read clock resultingin a phase hit and synchronization loss for the following circuit, thephase transition of the read clock should be slow enough to mitigate anyphase hits. This may be achieved by appropriately utilizing a phaselocked loop.

An exemplary communications system according to an embodiment of thepresent subject matter receives a constant bit rate signal at theantennas which is then provided to the receivers. The constant bit ratesignal received by the receivers is converted to a first baseband signaland a second baseband signal, respectively, and the demodulatorestimates a signal quality metric of the first baseband signal, comparesthe signal quality metric to a predetermined threshold, and providesalarm information to the CPU. The CPU evaluates the alarm informationprovided by the demodulator and provides switching control informationto the DADE circuit or other suitable switching circuit if the signalquality metric exceeds a predetermined threshold to thereby switch fromthe first baseband signal to the second baseband signal. The demodulatormay estimate the first signal quality metric by determining a firstdistance between a location of a first symbol of the first basebandsignal and a first predetermined ideal symbol location, determining asecond distance between a location of a second symbol of the firstbaseband signal and a second predetermined ideal symbol location, anddetermining an average distance for the first and second distancedeterminations whereby the average distance may be compared to thepredetermined threshold to generate the alarm information.

In a further embodiment, the demodulator may estimate the first signalquality metric by determining, for each of n symbols of the firstbaseband signal, a distance between a location of the symbol and anassociated predetermined ideal symbol location, determining a sum of thedistances for the n symbols, and dividing the sum by n whereby the sumdivided by n may be compared to a predetermined threshold to generatethe alarm information.

A communications system according to a further embodiment of the presentsubject matter receives a constant bit rate signal at the antennas whichare then provided to the receivers. The constant bit rate signalreceived by the receivers is converted to a first baseband signal and asecond baseband signal, respectively. A first demodulator estimates asignal quality metric of the first baseband signal, and a seconddemodulator estimates a signal quality metric of the second basebandsignal. The CPU compares the first signal quality metric to the secondsignal quality metric and provides switching control information to theDADE circuit or another suitable switching circuit if the signal qualitymetric exceeds a predetermined threshold to thereby switch from thefirst baseband signal to the second baseband signal. The firstdemodulator may estimate the first signal quality metric by determininga first distance between a location of a first symbol of the firstbaseband signal and a first predetermined ideal symbol location,determining a second distance between a location of a second symbol ofthe first baseband signal and a second predetermined ideal symbollocation, and determining a first average distance for the first andsecond distance determinations. The second demodulator may estimate thesecond signal quality metric by determining a third distance between alocation of a first symbol of the second baseband signal and a thirdpredetermined ideal symbol location, determining a fourth distancebetween a location of a second symbol of the second baseband signal anda fourth predetermined ideal symbol location, and determining a secondaverage distance for the third and fourth distance determinationswhereby the first average distance may be compared to the second averagedistance to generate the alarm information.

In a further embodiment, the first demodulator may estimate the firstsignal quality metric by determining, for each of n symbols of the firstbaseband signal, a distance between a location of the symbol and anassociated predetermined ideal symbol location, determining a first sumof the distances for the n symbols, and dividing the first sum by n.Similarly, the second demodulator may estimate the second signal qualitymetric by determining, for each of n symbols of the second basebandsignal, a distance between a location of the symbol and an associatedpredetermined ideal symbol location, determining a second sum of thedistances for the n symbols, and dividing the second sum by n wherebythe first sum divided by n is compared to the second sum divided by n togenerate the alarm information. Of course, the first and second signalquality metrics may be compared to predetermined thresholds rather thanto each other. For example, the first signal quality metric may becompared to a predetermined threshold and if the first signal qualitymetric is greater than a predetermined threshold then the second signalquality metric may be compared to a predetermined threshold. If thesecond signal quality metric is less than the predetermined threshold,then the CPU may provide switching control information to the DADEcircuit or another suitable switching circuit to thereby switch from thefirst baseband signal to the second baseband signal. However, if thesecond signal quality metric is greater than or equal to thepredetermined threshold, the first signal quality metric may be comparedto the second signal quality metric, and the CPU may provide switchingcontrol information to the DADE circuit or another suitable switchingcircuit to thereby switch from the first baseband signal to the secondbaseband signal if the first signal quality metric is greater than thesecond signal quality metric.

For example, when multipath occurs to a wide band spectrum, a slope withvarious steepness will occur to the affected spectrum while selectivefading may sweep through the spectrum. With proper adaptive slopeequalization, the slope in the spectrum may be corrected to a certainextent. When slope equalization reaches its correction limit, a slopealarm will be declared, which may be used to switch to a standbyreceiver. However, when the spectrum is narrow, relatively flat fadingoccurs during multipath occurrence. Thus, slope alarm may not be asreliable against multipath occurrences for narrow band signal.

By way of further example, FEC decoding generates syndromes forconvolutional decoders or corrected symbols in Reed Solomon decoderswhen transmission errors occur, which may be used to trigger receiverswitching. However, comparing the relative signal quality between thetwo may be a relatively slow process. Thus, switching from thisinformation may be too slow to ensure a reliable errorless switch forlower capacity data during fast fading periods. Thus, other events maybe used to initiate receiver switching, such as BER alarm, etc. Thus, inanother aspect of the present subject matter, a signal quality degradethreshold to trigger the switching may be set such that an alarm will betriggered before any error appears at an FEC decoder output.

In a multipath environment, selective fading may sweep through areceiver antenna with progressive speed. However, with a 1+1 protectedFD system 700 according to an embodiment of the present subject matter,the two RF carriers are sufficiently separated in frequency and thus,the signal distortion will occur to one carrier and then to the nextcarrier, but not both simultaneously. Furthermore, with a 1+1 protectedSD system 600 according to an embodiment of the present subject matter,the two receive antennas 604, 606 are properly displaced and thus,selective fading will affect one of the two antennas, but not bothsimultaneously. Thus, with FD or SD systems or combinations thereof,error free data transmission can be ensured even during severe pathdisturbance period.

In a further aspect of the present subject matter, the system mayincorporate Adaptive Time Domain Equalization (ATDE) and/or forwarderror correction (FEC) such that the alarm information and subsequentreceiver switching will occur before any error occurs to output dataduring signal path fading (selective or flat fading). Thus, inconjunction with a DADE circuit, switching from a degraded activechannel to a good standby channel may be completed without any error.

Although embodiments of the present subject matter have been describedwith application to 4 QAM, it is to be understood that the presentsubject matter is applicable for M-ary QAM systems and M-state PSKsystems, i.e., embodiments of the present subject matter may be utilizedin any M-ary QAM demodulator (M=4, 8, 16, 32, 64, 128, 256, etc.) andmay be utilized in any M-state PSK demodulator (M=2, 4, 8, 16, 32, 64,128, 256, etc.). Furthermore, it is to be understood that embodiments ofthe present subject matter may also be utilized in any non conventional2 dimensional (I-Q) modulation.

As shown by the various configurations and embodiments illustrated inFIGS. 1-7, the system and method for receiver switching based on signalquality estimation according to the present subject matter may beutilized for selecting antennas, channels or receivers in a multipathenvironment without incurring any errors in a received signal.

While preferred embodiments of the present subject matter have beendescribed, it is to be understood that the embodiments described areillustrative only and that the scope of the invention is to be definedsolely by the appended claims when accorded a full range of equivalence,many variations and modifications naturally occurring to those of skillin the art from a perusal hereof.

1. A system, comprising: a first converting circuit for converting asignal from the first receiver to a first baseband signal; a secondconverting circuit for converting the signal from the second receiver toa second baseband signal; a first estimating circuit for estimating afirst signal quality metric of the first baseband signal; a secondestimating circuit for estimating a second signal quality metric of thesecond baseband signal; a comparing circuit for comparing the firstsignal quality metric to a predetermined threshold to determine if thefirst signal quality metric meets a signal quality; and a switchingcircuit for switching from the first baseband signal to the secondbaseband signal based on the comparison of the first signal qualitymetric to the predetermined threshold and based on other signal qualityinformation; wherein the estimating circuit comprises: a determiningcircuit for outputting, for each of n symbols of the first basebandsignal, a first signal that is a function of a distance between alocation of the symbol and an associated predetermined ideal symbollocation; a summing circuit for receiving the first signals andoutputting a second signal that is representative of a sum of the firstsignals for the n symbols; and a dividing circuit for receiving thesecond signal and outputting a third signal that is representative ofdividing the second signal by n.
 2. The system of claim 1, wherein theother signal quality information comprises intermediate frequency (IF)spectrum information, intermediate frequency (IF) slope information,bit-error-rate (BER) information, signal degradation information, orsignal outage information.
 3. The system of claim 2, wherein the signaldegradation information is provided by a forward-error-coding (FEC)process.
 4. The system of claim 1, wherein the switching circuit isconfigured to switch from the first baseband signal to the secondbaseband signal according to a switching request priority associatedwith the comparison of the first signal quality metric to thepredetermined threshold and other signal quality information.
 5. Thesystem of claim 4, wherein the switching priority comprisesbit-error-rate (BER), frame loss, signal degradation, intermediatefrequency (IF) slope, or low receiver signal level (RSL) sensed at the aradio antenna port.
 6. The system of claim 1, wherein the switchingcircuit comprises a differential absolute delay equalizer (DADE)circuit.
 7. A method, comprising: (a) converting a signal from the firstreceiver to a first baseband signal; (b) converting the signal from thesecond receiver to a second baseband signal; (c) estimating a firstsignal quality metric of the first baseband signal; (d) comparing thefirst signal quality metric to a predetermined threshold to determine ifthe first signal quality metric meets a signal quality; (e) switchingfrom the first baseband signal to the second baseband signal based onthe comparison of the first signal quality metric to the predeterminedthreshold and based on other signal quality information; wherein thestep of estimating the first signal quality metric comprises the stepsof: (i) determining, for each of n symbols of the first baseband signal,a distance between a location of the symbol and an associatedpredetermined ideal symbol location; (ii) determining a first sum of thedistances for the n symbols; and (iii) dividing the first sum by n. 8.The method of claim 7, wherein the other signal quality informationcomprises intermediate frequency (IF) spectrum information, intermediatefrequency (IF) slope information, bit-error-rate (BER) information,signal degradation information, or signal outage information.
 9. Thesystem of claim 8, wherein the signal degradation information isprovided by a forward-error-coding (FEC) process.
 10. The system ofclaim 7, wherein the switching circuit is configured to switch from thefirst baseband signal to the second baseband signal according to aswitching request priority associated with the comparison of the firstsignal quality metric to the predetermined threshold and other signalquality information.
 11. The system of claim 10, wherein the switchingpriority comprises bit-error-rate (BER), frame loss, signal degradation,intermediate frequency (IF) slope, or low receiver signal level (RSL)sensed at the a radio antenna port.
 12. A system, comprising: a meansfor converting the signal from the first receiver to a first basebandsignal; a means for converting the signal from the second receiver to asecond baseband signal; a means for estimating a first signal qualitymetric of the first baseband signal; a means for comparing the firstsignal quality metric to a predetermined threshold to determine if thefirst signal quality metric meets a signal quality; a means forswitching from the first baseband signal to the second baseband signalbased on the comparison of the first signal quality metric to thepredetermined threshold and based on other signal quality information;wherein the means for estimating the first signal quality metriccomprises: (i) determining, for each of n symbols of the first basebandsignal, a distance between a location of the symbol and an associatedpredetermined ideal symbol location; (ii) determining a first sum of thedistances for the n symbols; and (iii) dividing the first sum by n.